The present invention relates to wireless digital communication systems. More particularly, the present invention relates to communication stations which employ code-division multiple access (CDMA) technology wherein the station has multiple antennas for increasing the capacity of the CDMA system.
Over the last decade consumers have become accustomed to the convenience of wireless communication systems. This has resulted in a tremendous increase in the demand for wireless telephones, wireless data transmission and wireless access to the Internet. The amount of available RF spectrum for any particular system is often quite limited due to government regulation and spectrum allotments. Accordingly, the need to utilize one's allocated RF spectrum efficiently is desired.
CDMA communication systems have shown promise in the effort to provide efficient utilization of the RF spectrum. At least one brand of CDMA systems, Broadband Code Division Multiple Access™ or B-CDMA™ communication systems available from InterDigital Communications Corporation, permit many communications to be transmitted over the same bandwidth, thereby greatly increasing the capacity of the RF spectrum. In B-CDMA™ brand communication systems, an information signal at the transmitter is mixed with a pseudo random “spreading code” which spreads the information signal across the entire bandwidth which is employed by the communication system. The spread signal is upconverted to an RF signal for transmission. A receiver, identified by the pseudo random spreading code, receives the transmitted RF signal and mixes the received signal with an RF sinusoidal signal generated at the receiver by a first-stage local oscillator to downconvert the spread spectrum signal. The spread information signal is subsequently mixed with the pseudo random spreading code, which has also been locally generated, to obtain the original information signal.
In order to detect the information embedded in a received signal, a receiver must use the same pseudo random spreading code that was used to spread the signal. All signals which are not encoded with the pseudo random code of the receiver appear as background noise to the receiver. Accordingly, as the number of users that are communicating within the operating range of a particular communication station increases, the amount of background noise also increases, making it difficult for receivers to properly detect and receive signals. The transmitter may increase the power of the transmitted signal, but this will increase the noise (interference) as seen by other receivers.
Applicants have recognized the need to decrease the amount of interference in order to increase the capacity (number of users) of the CDMA system.